The present invention relates in general to air-cleaning apparata and in particular to such apparata which also achieve heat exchange and heat transfer.
Industrial exhaust poses a major problem from the standpoint of pollution, and with the promulgation of federal rules and regulations and clean air standards, many efforts have been focused on how to reduce the level of contaminants from the effluent wastes. A related problem occurs in the area of restaurant exhaust from areas such as grills and broilers. Restaurants, expecially fast-food restaurants, generate large amounts of grease-laden exhaust from the frying and broiling of the various food items which are prepared. This grease-laden exhaust is typically drawn up through a kitchen hood and exhaust duct combination to the atmosphere by a large ventilating fan arrangement. The problem with this type of design is that the grease from the exhaust collects on the internal surfaces of the hood, the exhaust duct and the fan arrangement and must be periodically cleaned. The frequency of required cleaning depends upon the volume of exhaust and the amount of grease, but a cleaning frequency of once a month is very often advisable in order to assure that grease levels will not reach the point where a fire occurring in the kitchen area will spread to and ignite this grease. Unfortunately, the thorough cleaning of the grease from the hood, exhaust duct and ventilating fan arrangement is difficult and is often quite time consuming and represents significant downtime to the particular restaurant. As a result of the inconvenience represented by the cleaning procedure, removal of the grease from the internal surfaces is not always performed when required. A somewhat predictable consequence of this lack of proper and periodic cleaning is that the hood, exhaust duct and fan arrangement will become heavily grease coated as well as producing a noticeable odor. Significant fire disasters have occurred where a grease fire in the kitchen exhaust duct was attributed to be a cause or at least a contributing factor to the spreading of the fire.
Recently efforts have been made to chemically treat such grease-laden exhaust so that the grease is emulsified and its buildup on internal surfaces is reduced. By using a mist-like spray of a chemical and water mixture directed around the inside of the exhaust duct between the hood and ventilating fan, the emulsified grease is either drained off in liquid form or may be exhausted in vapor form. Although this method has proved effective to prevent a majority of the grease buildup in the exhaust duct and on the ventilating fan, there remain certain shortcomings and disadvantages.
The spray arrangement just mentioned must have its volume of chemical solution governed by the volume of exhaust and the amount of grease, and to be effective, the sprayer must direct a mist against the full inside perimeter of the exhaust duct so that all surfaces can be cleaned of grease. This poses a very exacting requirement on the design of the spray nozzle head as well as its positioning within the exhaust duct. However, even with well-designed and positioned spray nozzle heads, the grease which is emulsified by the chemical solution spray does not represent 100 percent of the grease flowing through the exhaust duct and the emulsified grease which is exhausted in vapor form, as well as the grease which is not chemically treated, still contributes to the pollution of the atmosphere. Another very significant factor, especially with energy consideration concerns, is the amount of heat which is generated by the restaurant grills and broilers and is lost up the exhaust duct. Any attempt to capture this heat and reuse it for general heating of the structure runs headlong into the related problems of grease accumulation. If a heat exchanger is employed, once the convecting surfaces, such as the fins, collect a thin film of grease, the heat-transfer properties of such convecting surfaces are reduced to the point that the heat exchanger becomes highly inefficient. This grease buildup acts as a thermal insulator and prevents efficient heat transfer from the hot exhaust gas to a heat-absorptive fluid circulating through the heat exchanger. Related problems involve how to extract a majority of this exhaust duct heat so that system efficiency is high and how can the complete system be maintained in a virtually grease-free manner while still not exhausting harmful pollutants (such as emulsified grease) into the atmosphere. A collateral benefit of grease removal is that the accompanying odor will also be eliminated. The shortcomings and disadvantages discussed above are overcome by the apparatus of the present invention which is disclosed herein.